Condensed Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenues	$ 570,222	$ 592,349	$ 1,125,405	$ 1,167,644
Costs, expenses and other operating
Cost of sales (exclusive of items shown separately below)	451,088	474,388	889,559	929,168
Depreciation, depletion and amortization	26,524	30,549	51,797	60,252
Accretion on asset retirement obligations	5,137	6,993	10,274	13,985
Amortization of sales contracts, net	11	3,248	76	6,299
Change in fair value of coal derivatives and coal trading activities, net	(8,400)	15,138	(21,381)	11,724
Selling, general and administrative expenses	25,209	24,756	49,298	50,704
Costs related to proposed joint venture with Peabody Energy	3,018	0	3,018	0
Loss on sale of Lone Mountain Processing, LLC	(4,304)	0	(4,304)	0
Other operating income, net	(3,239)	(7,318)	(4,889)	(14,250)
Costs, expenses and other operating	503,652	547,754	982,056	1,057,882
Income from operations	66,570	44,595	143,349	109,762
Interest expense, net
Interest expense	(4,375)	(5,050)	(8,807)	(10,445)
Interest and investment income	2,088	1,552	4,231	2,825
Interest expense, net	(2,287)	(3,498)	(4,576)	(7,620)
Income before nonoperating expenses	64,283	41,097	138,773	102,142
Nonoperating (expenses) income
Non-service related pension and postretirement benefit (costs) credits	(1,336)	68	(3,102)	(1,235)
Net loss resulting from early retirement of debt and debt restructuring	0	(485)	0	(485)
Reorganization items, net	(16)	(740)	71	(1,041)
Nonoperating expenses	(1,352)	(1,157)	(3,031)	(2,761)
Income before income taxes	62,931	39,940	135,742	99,381
Provision for (benefit from) income taxes	91	(3,366)	161	(3,910)
Net income	$ 62,840	$ 43,306	$ 135,581	$ 103,291
Net income per common share
Basic earnings per common share (in dollars per share)	$ 3.80	$ 2.15	$ 7.97	$ 5.03
Diluted earnings per common share (in dollars per share)	$ 3.53	$ 2.06	$ 7.45	$ 4.81
Weighted average shares outstanding
Basic weighted average shares outstanding (in shares)	16,543	20,156	17,018	20,529
Diluted weighted average shares outstanding (in shares)	17,781	21,036	18,190	21,456
Dividends declared per common share (in dollars per share)	$ 0.45	$ 0.40	$ 0.90	$ 0.80